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Chocolate Layer: Heat oven to 350 degrees.
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Line an 8 inch baking pan with foil.
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Grease foil.
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Whisk together flour, baking powder and salt in a medium bowl.
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Set both aside.
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Melt butter and chocoalte in a large microwave bowl on high 1 1/2 to 2 minutes until melted.
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Transfer to a large mixer bowl and beat in sugar and eggs on medium high speed until well combined, 2 to 3 minutes.
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Reduce speed to low; beat in flour mixture just until combined.
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Fold in pecans.
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Pour batter into prepared pan.
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Bake until just set in middle and sides just begin to pull away from the pan, 22 to 26 minutes.
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Cool completely on wire rack.
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Buttercream Topping: Beat butter, confectioner's sugar, cream and vanilla in a large mixer bowl on medium speed until smooth, about 1 minute.
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Reduce speed to low and beat until fluffy and smooth, 1 to 2 minutes more.
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Chocolate Drizzle: Stir chocolate and butter together in small bowl until smooth.
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Cool slightly and transfer to a heavy duty plastic storage bag.
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Lift chocolate layer out of pan.
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Peel off foil.
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Transfer to a flat surface lined with waxed paper.
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Spread buttercream evenly over top of cookie.
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Snip one tiny corner of plastic bag and drizzle chocolate glaze over top as desired.
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Let stand until chocolate is set, 30 to 60 minutes.
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Cut into 3 1/2 x 1 inch bars.